The present invention relates to an infrared radiator.
A resistor element heated by electric current is generally used in gas analyzers based on the measurement of infrared absorption in absorption bands characteristic of the gases under measurement. The advantages of such a thermal radiation source include a low price, fast starting and a wide-band radiation output consistent with Planck's law.
According to Planck's law, the radiation output of a thermal radiator is strongly dependent on the temperature of the radiating surface. For instance, when the carbon dioxide content in the expired air of an anesthetized patient is being measured with 4.27 .mu.m wavelength, the output of the radiation source increases over 4% as its temperature rises from 900 K.fwdarw.0 K. Such a temperature change may occur e.g. when the ambient temperature of the gas analyzer changes by 10.degree. C.
As is known, the effect of the change in radiation output on the measurement accuracy of the analyzer can be eliminated by using dual-beam optics in the analyzer. However, this requires moving mechanical parts, which reduce the analyzer's reliability and life span and increase its price.
The temperature changes in the infrared source can be taken into account by periodically performing a zero point adjustment, which, however, interrupts the analyzer's normal operation, thereby impairing patient safety. During the transition after the starting of the analyzer, zero point adjustments may have to be performed at intervals of a few minutes intervals.
From the point of view of construction, operation and measurement accuracy of the gas analyzer, it would therefore be desirable to stabilize the temperature of the infrared source accurately as possible.
There are several infrared radiators known in the art, one of which is presented in PCT publication W093/09412. To measure the mean temperature of the heating element, this solution uses a temperature detector in conjunction with the heating element acting as an infrared source, the detector being thermally coupled to the infrared source but electrically isolated from it. In addition, a regulator is used which, based on the measured temperature, adjusts the power supplied to the heating element so that the temperature of the heating element remains at the desired level.
On the other hand, U.S. Pat. No. 4,620,104 presents a solution in which the surface to be heated with electric current is placed on a platelike base, the opposite surface of which is provided with a coating acting as a temperature detector. Based on the feedback obtained from the coating, the power supplied to the radiation source is adjusted so as to keep the radiation source at a constant temperature.
Both of the aforementioned solutions have the drawback that measuring the temperature of the radiation source requires a separate temperature detector along with the mechanical fittings, electrical insulation and conductors involved. As the temperature of the radiation source rises to a high level during operation, the temperature detector and its electrical and mechanical fittings are subjected to both large and rapid temperature changes that impair their reliable implementation and reduce the reliability of the infrared source.
U.S. Pat. No. 4,499,382 presents an infrared radiator in which the temperature of the radiation source is kept constant by supplying the infrared source with a power level such that the resistance depending on the temperature of the resistor core coil remains constant. Since the temperature coefficient of the resistance of the resistor wire generally used, made of chrome nickel alloy, is very small, typically under 100 micro-ohms/.degree.C., the use of this method requires very accurate resistance measurement that is independent of temperature.